When Frost Threatens Plants

There’s a May frost in the forecast. What are the damage possibilities and how can we protect our plants?

Different plants will be affected differently.

  1. Perennial flowers, rhubarb, strawberries and other established plants that are actively growing can usually weather temperatures between 30 and 32 degrees with little injury. They’ll often be fine even if the temperature dips between 28 and 30 degrees. Below 28 degrees has the potential for damaging foliage, but perennial flowers and plants have the capacity to regrow from the base. Cover as a precaution if you are able.
  2. Plants that were greenhouse-grown and recently planted are very tender and susceptible to injury at 32 degrees. This includes annuals that have been planted in containers, window boxes and planters. Especially susceptible to frost injury are warm-loving plants like coleus, impatiens, tomato, pepper, eggplant, cucumber, squash and melons. These should be protected.
  3. Vegetables that have emerged in the garden vary on frost sensitivity, depending on type. Cool-season crops like cabbage, cauliflower, broccoli, onions, carrots, lettuce, peas, spinach and potatoes can tolerate temperatures down to about 28 degrees. Warm-season crops like tomatoes and peppers are injured at about 32 degrees and should be protected.
  4. Established trees and shrubs can tolerate temperatures to 28 degrees.
  5. Flowers on fruit trees in full bloom can be injured around 25 to 28 degrees. If some of the flowers are tightly closed, they’ll survive frost better than those completely open. Fruit can be reduced as a result.

How to protect plants from freezing temperatures:

  1. Water the soil and the foliage before nightfall. Dry plants are frost-injured more severely than moist plants. A layer of water on leaves helps protect from freezing. Citrus growers down South often turn on sprinklers on freezing nights.
  2. Move containers into the garage, or along the house foundation.
  3. If it’s not possible to move plants indoors, cover with cloth, blankets, paper or cardboard. Plastic does not work well, because it tends to transmit cold, often damaging foliage where the plastic touches. If using plastic, prop it up with wooden stakes or poles so the plastic doesn’t contact foliage.
  4. Often cold is accompanied by wind, which makes covering plants difficult. Wind frequently dies down in evening or after midnight. The lowest temperatures causing freezing damage often occur between about 4:00 and 7:00 a.m.
  5. For plants that cannot be covered or moved, spraying them with the garden hose during the night can help, as citrus growers have found.

Temperature in various locations is bound to fluctuate from the “official” temperature. Often just a variance of a few degrees can make a big difference in injury or survival.

Here’s hoping all our plants survive unfrosted.